Improve UI and aesthetics

[joshenx]

UI: Tab Update!

New Features:

• Appointments have two tabs, Upcoming and Archived

• Application has a new Logo
  

• Text buttons have been replaced with icons

• Selected cards have clearer markers

• Appointment cards now split Patient and Appointment info

Current UI

[codecov-commenter]

Codecov Report

Merging #121 (dd9f7f2) into master (e5b3a2f) will decrease coverage by 0.23%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #121      +/-   ##
============================================
- Coverage     61.62%   61.38%   -0.24%     
  Complexity      491      491              
============================================
  Files            94       94              
  Lines          1793     1800       +7     
  Branches        190      190              
============================================
  Hits           1105     1105              
- Misses          629      636       +7     
  Partials         59       59              

Impacted Files	Coverage Δ
...n/java/seedu/address/commons/core/GuiSettings.java	69.23% <ø> (ø)
src/main/java/seedu/address/ui/MainWindow.java	0.00% <0.00%> (ø)
src/main/java/seedu/address/ui/UiManager.java	0.00% <0.00%> (ø)

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[didymental]

LGTM